Many communication and radio-navigation systems employ receivers wherein signals are tracked coherently. Coherent tracking receivers seek to preserve the phase of the signals when knowledge of the phase may be necessary for data demodulation or for improving receiver performance. For example, a receiver for the satellite-based Global Positioning System (GPS) performs coherent integration of the signals to enable coherent tracking in order to improve receiver sensitivity. Coherent tracking requires that the crystal oscillator generating the receiver clock be relatively stable during the coherent integration interval. However, many receivers use TCXO or VC-TCXO as their reference oscillator because the temperature compensation feature of the TCXO or the VC-TCXO reduces the frequency uncertainty of the reference oscillator and thus, the search range required to find signals of interest during acquisition. As such, compensating for temperature changes to support fast acquisition may adversely affect the results of coherent integration in tracking if a change in the oscillator frequency is induced by the compensation network during the integration interval. For example, initiating temperature compensation or changing the compensation in the TCXO or the VC-TCXO may introduce phase, frequency, and/or frequency-rate discontinuities to the receiver oscillator. Such oscillator discontinuities, if they occur during the coherent integration period of the GPS receiver, may degrade tracking performance and reduce receiver sensitivity. To avoid this problem temperature compensation may be initiated between periods of coherent integrations. However, for multi-channel GPS receivers where coherent integration periods of the multiple tracking channels may overlap, it may be difficult to find any gap in the coherent integration periods when no channel is running coherent integration to initiate temperature compensation so as to avoid generating oscillator discontinuities. Even non-coherent tracking receivers are not immune from receiver oscillator discontinuities. Non-coherent tracking receivers do not preserve the phase of the signal for data demodulation and are generally more tolerant of receiver oscillator discontinuities. However, phase, frequency, and/or frequency-rate discontinuities on the receiver oscillator may also degrade tracking performance of non-coherent receivers if the discontinuities are too great.
Therefore, it is desirable to have a single TCXO or VC-TCXO that combines the advantage of an oscillator with a reduced range of frequency error over temperature with the flexibility to generate an oscillator without discontinuities when no temperature compensation is desired.